Are seed oils actually bad for you?
Mostly no. In controlled trials the linoleic acid in seed oils doesn't raise inflammation, and people with more of it in their blood tend to have less heart disease and diabetes, not more.
The case against seed oils goes like this: they’re loaded with omega-6 linoleic acid, your body converts that into arachidonic acid, and arachidonic acid feeds inflammation — so canola, soybean, and sunflower oil must be quietly driving heart disease and diabetes. It’s a tidy story. The trouble is that when researchers actually feed people more linoleic acid and measure their blood, the inflammation never shows up.
Two systematic reviews of randomized trials — one in healthy adults, the other pooling 30 trials across nearly 1,400 people — found no meaningful change in C-reactive protein, interleukin-6, TNF, or the other usual inflammatory markers when linoleic acid intake went up. The omega-6-to-inflammation step is mostly textbook biochemistry that doesn’t play out in living humans: only a tiny fraction of linoleic acid is converted to arachidonic acid, and that molecule seeds anti-inflammatory compounds too.
What the outcome data show
The Cochrane review of omega-6 trials — 19 randomized studies, over 6,400 people — found that eating more omega-6 does little or nothing to overall mortality, may modestly cut heart attacks, and reliably lowers total cholesterol. Not the fingerprint of a poison. On the observational side, a pooled analysis of 30 cohorts (about 68,000 people) found that those with more linoleic acid in their blood had lower rates of cardiovascular disease and cardiovascular death. A separate pooling of 20 studies (nearly 40,000 people) tied higher linoleic acid to less type 2 diabetes. The blood biomarker everyone worries about points the friendly direction.
Where the real caveat lives
Two legitimate concerns hide under the panic. Oil that’s repeatedly heated and reused for deep-frying does generate oxidized byproducts — a cooking-method problem, not a reason to fear the oil in your salad dressing. And most seed oil in the American diet rides in on ultra-processed foods, which are worth eating less of for reasons unrelated to linoleic acid. So cook with the oil you like, skip the week-old fryer grease, and aim your suspicion at the chips, not the canola.